0:07Skip to 0 minutes and 7 secondsIf you remember back to an earlier video where we looked at the external surface of the brain using the plastic model, you might remember that one of the features that I pointed out to you was that the brain had a folded surface. Now this surface, which is technically known as the cortex or the cerebrum, contains many of the very important neurons that underpin function. However, if we look in lower mammals-- so rodents, for example, like rats and mice-- we see that they don't have brains with that folded appearance. They have smooth brains. And we might ask ourself why this is. And the answer is it all comes down to numbers.

0:45Skip to 0 minutes and 45 secondsSo if you think about what's happened over evolutionary time, as we've had to increase numbers of neurons in the brain, we've had to find somewhere to put them. And here I've got a couple of pieces of papers to try and illustrate that point to you. So these circles represent neurones, cell bodies of neurones. So if you imagine that as we increase in number, we have to put them somewhere, one option is just to increase the surface area over which they exist. But you can imagine that as you increased by size even further, you would end up with an enormous surface of the brain.

1:26Skip to 1 minute and 26 secondsSo an alternative strategy is to change the shape of that surface and to fold it like this. If we do this, what I've just created is the groove-- remember, the sulcus and here, two of our gyri-- which form the surface of the human brain. And of course, what you can obviously see is how much less surface this takes up than just having a flat plane of neurons. Although the depth is greater, that can be accommodated in the increased volume of the brain.

Neurone arrangement - 2

This video highlights the “folded” appearance of the brain’s surface and how it has come to be organised in this way.

In February 2016, a group of Harvard scientists built a model brain containing a material on the outside that could be made to swell up. When they did this they saw that the initially smooth brain surface developed folds and a shape that was remarkably reminiscent of the human brain. You can see a video of the swelling brain here.